1. Technical Field
The present invention relates to a growth-modified thermal oxidation process for forming thin (e.g., &lt;250 .ANG.) oxides for use in silicon integrated circuits.
2. Description of the Prior Art
A major impediment to providing ultra-large scale (&gt;10 million devices/chip) silicon-based integrated circuits, based on sub-micron design rules, has been the inability to grow extremely thin oxides which exhibit the necessary low defect density (D.sub.o), low charge trapping density (Q.sub.it), and atomically sharp, stress-free Si/SiO.sub.2 interface. It has been determined that the characteristics of the silicon nucleation surface play a significant part in determining the characteristics of the grown oxide. Preoxidation cleaning of the silicon surface, as described in detail in an article entitled "Cleaning Solutions Based on Hydrogen Peroxide for use in Silicon Semiconductor Technology", by W. Kern et al. appearing in RCA Review, Vol. 31, June 1970, pp. 187-206, has been found to improve the nucleation surface. In particular, the Kern et al. article discusses the use of various solutions, including H.sub.2 O.sub.2 -NH.sub.4 OH and H.sub.2 -H.sub.2 O.sub.2 -HCl to prepare a silicon surface for oxidation. This preoxidation cleaning has become an integral part of oxidation technology. Such cleaning processes, however, cannot alleviate the problems of growth-induced oxide defects and interfacial stress between the silicon substrate and the grown oxide.
In an alternative method of improving the oxide quality, a small amount of a chlorine-bearing vapor may be added to the oxidant during growth. This particular work is described in detail in the reference entitled "The Use of 1.1.1-Trichloroethane as an Optimized Additive to Improve the Silicon Thermal Oxidation Technology", by E. J. Janssens et al. appearing in the Journal Electrochem. Society, Vol. 125, No. 10, October 1978, pp. 1696-1703. The addition of the chlorine-bearing species (for example, hydrochloric acid (HCl), trichloroethane (TCA), trichloroethene (TCE) has been found to reduce the concentration of mobile ions, oxide defects, and surface charge density in the oxide. However, such oxides still exhibit growth-induced defects, alkali metal contamination and interfacial stress problems.
A stacked film of SiO.sub.2 /Si.sub.3 N.sub.4 /SiO.sub.2 with a total thickness of between 100-200 .ANG. has been demonstrated to obtain a relatively low defect density (D.sub.o =0.5 cm.sup.-2). See, for example, the article entitled "A 100 .ANG. Thick Stacked SiO.sub.2 /Si.sub.3 N.sub.4 /SiO.sub.2 Dielectric Layer for Memory Capacitor" by T. Watanabe et al. appearing in the Proceedings of the International Reliability Physics Symposium, 1985, pp. 18-23. However, the Si.sub.3 N.sub.4 /SiO.sub.2 interface is known to exhibit a high density of interface charge trapping states, Q.sub.it, which cannot be removed by annealing since the nitride is impervious to the oxidizing species. This tri-level structure is unsuitable as a gate dielectric in MOS integrated circuits, as these interface states may cause charge-induced shifts in threshold voltage, as well as a reduction in channel conductance during device operation.
Thus, a need remains in the prior art for a method of forming a thin oxide which exhibits the characteristics required for use as a gate dielectric in sub-micron MOS device structures.